At one time, pavement design seemed to be a fairly simple process. Materials and prices were fairly stable, and many engineers and other people in the business could often tell at a glance what structure was best for any particular application.
Today, a host of external factors have combined to make pavement engineering considerably more complex. The specifier today must take into account such diverse considerations as material availability and optimum utilization of scarce natural resources. With the rapidly increasing costs of asphalt, labor and other components, it is becoming critically important that each pavement project be carefully engineered not only for adequate strength and sufficient durability, but for a minimum cost and optimum value as well.
The basic procedures and formulas needed to make these decisions are reasonably well established and are commonly accepted. But until now, they have required long and tedious calculations, using charts, graphs and a good deal of trial and error. A simpler procedure is needed.
The present invention was designed to meet this need. It provides a simple, practical way to (1) determine the structural requirements of a particular application, according to traffic and soil conditions, (2) determine the various alternative combinations of surface, base and sub-base that will meet these structural requirements, and (3) compare the real cost of these alternatives in terms of cost/square yard at current materials prices, to find the most economical combination for these structural requirements. Most important, the invention comprises a simple, easy to use calculator that enables the user to make these determinations quickly, accurately and efficiently.
Pavement design is still largely an art, and requires a considerable degree of engineering judgment.
At the same time, however, there is a body of commonly accepted formulas and relationships that may be used to estimate the proper thickness of pavement for any given application, and these are incorporated in this system. They are reasonably valid for typical situations and can be adjusted to fit unusual conditions. In order to make sound judgments, a design still requires a knowledge of pavement behavior and characteristics.
The flexible pavement engineering system of this invention is a tool to assist the engineer in making these judgments. Used in this manner, it will provide a basis for making comparisons using current prices to find the one structural alternative that offers the optimum combination of performance and cost.
The system follows standard engineering procedure. Its strength is in its simplicity and its ability to quickly and easily give answers to complex computations.
Initially, a determination is made of the proper structural design coefficient for a particular application expressed in terms of an SN number. This in turn requires a reasonable prediction of the traffic load that the pavement will be subjected to, expressed as a Design Traffic Number (DTN), and also the properties of the soil on which the pavement is to be constructed, expressed as a CBR (California Bearing Ratio) number.
The calculator provides a simple way of determining the former.
From these two numerical values, the calculator will directly compute the Structural Number, or SN, for the particular application. It will then provide various alternative material combinations that will yield this SN and assist in determining the actual cost per square yard of each of these alternatives.